Motor vehicles in motion are subjected to various forces which oppose their advancement. These forces are in particular the solid friction forces, due to the contact of the wheels with the ground, and the aerodynamic forces, due partly to friction of the air on the vehicle bodywork panels, and partly to the pressure exerted mainly on the front and rear of the vehicle.
At high speed, these aerodynamic forces become dominant with respect to the other forces and play a major role on the energy consumption of the vehicle. Some of the aerodynamic forces are generated by the depressions created by the separations, the recirculations and the longitudinal vortices of the air at the rear of the vehicle
Motor vehicles are therefore generally fitted with aerodynamic systems designed to facilitate the flow of air near the vehicle bodywork parts, more precisely to avoid the turbulence which generates a loss of energy and therefore, higher energy consumption by the vehicle.
It is known that this turbulence can be generated in the boundary layer separation regions marked by incidence breaks of vehicle bodywork parts. These breaks, also called trailing edges or boundary layer separation lines, correspond to breaks in the part shape (change of curve, projecting shape, end of part) leading to incidence breaks of the air flow relative to the panel. These boundary layer separation regions generate, downstream, turbulent zones where a depression is created when the vehicle is moving. This turbulent zone comprises a shear layer and a main returning vortex. Such zones are found for example near the upper rear end of the vehicle, formed by the transverse connection between the roof and the top of the tailgate, or near the rear window pillars or in the lower part of the rear bumper or at the rear bumper end caps. Turbulent or depression zones may also be found near the upper front end of the vehicle, formed by the transverse connection between the roof and the top of the windscreen, near the windscreen pillars, the front or rear wings or the bodywork parts around the front or rear wheel arches.
To improve the air flow in these turbulent zones, especially at the rear of the vehicle, an attempt is made to refine or extend these trailing edges by adapting the shape of the tailgate or the roof. Conventional systems consist mainly of spoilers, flaps or deflectors placed in these areas at the rear of the vehicle.
However, such parts do not eliminate the turbulence completely, and are relatively aesthetically restricting.
Aerodynamic devices are also known, from document EP1740442B for example, which consist of one or more fins forming at least one vortex generator and generally arranged upstream of a boundary layer separation line of a bodywork element of the motor vehicle. These fins can generally be moved between a retracted position and a projecting position in which they act on the turbulent structures generated around the bodywork elements of the vehicle.
However, such devices are faced with problems of size, appearance and integration on the bodywork elements. For example, the fins must always be retracted under certain conditions, which requires complex technical solutions to move the fins.
Moreover, while these fin systems are well suited when the angle of the boundary layer separation region is small, this is not the case when this angle is large. This angle is the angle defined by the break in the bodywork part shape, for example the angle between the roof and the rear window.
An aerodynamic system to limit losses in the shear layer is also known, from document EP 1 873 044 for example, without imposing special shapes on the bodywork parts. Such a system comprises electromechanical means arranged near a boundary layer separation region of the vehicle. These means are able to convert electricity into pressure waves in order to create a periodic air jet. This jet reduces the structures produced in the shear layer and decreases the intensity of the vortex produced downstream.
However, such a solution requires very precise positioning to be efficient, and is difficult to adapt to different types of vehicle and/or to different vehicle driving speeds.